1. FIELD OF THE INVENTION
This invention relates to improved electrically conductive compositions (pastes) employed in the manufacture of power surge resistor devices employed for power surge protection applications. The compositions are capable of being deposited by screen printing on alumina and similar ceramic substrates and the resultant surge resistors are characterized in their ability to dissipate repeated voltage surges equal in voltage and duration to surges created by lightning with less than 0.05% change in sheet resistance.
2. PRIOR ART
Voltage surge protective devices known in the art encompass a variety of materials, including, for example, epoxy/silicon, copper-nickel combinations, zinc oxide and zinc oxide with other oxide additives, forms and designs, and exhibit a variety of operating characteristics.
Power surge resistor devices of interest here are electrical devices manufactured with formulated resistor pastes or compositions based on palladium/silver powders or particles, glass and dopant(s). When a surge resistor device is wired into an electrical circuit, the electrical circuit is protected from damage by excess surges of line voltage such as may occur when the power line connected to a circuit is struck by lightning. Such devices are commonly wired into the electronic circuitry of television receivers, telephones, data processing equipment, computer elements and the like as a means for lightning strike and voltage surge protection.
Manufacturers of such protective devices are constantly searching for surge resistor pastes which will provide improved surge protection devices, particularly with respect to repeated voltage surges or lightning strikes. Informal standards set by industry and Underwriters Laboratories require the devices to withstand 20-200 multiple pulse surges at 2,500 volts and 1,000 volts, respectively, with less than 2% shift in sheet resistance.
Commercially available resistor pastes are also deficient in exhibiting temperature coefficient of resistance values (TCR) which are higher than desired by the device manufacturers. The relationship between the resistance change as a function of temperature is designated as the TCR of the protective device and defined as parts per million parts change per degree centigrade of temperature change, i.e., ppm/.degree.C. Lower TCR values over a designated range of temperatures are now required for commercial acceptance as compared to TCR values of pastes sold 5-7 years ago.
The resistor pastes of this invention provide surge protection devices which have a sheet resistance of 0.1 to about 20 ohms per square and are capable of exceeding present industry standards in respect of multiple pulse surges as noted above and in meeting TCR values which are less than .+-.50 ppm/.degree.C.
Resistor pastes based on palladium/silver combinations and glass have been known and used by practitioners for many years. See, for example, U.S. Pat. No. 3,639,274 issued Feb. 1, 1972 to Ivan L. Brandt et. al. The resistor pastes described in that patent are for the manufacture of ordinary resistors, not surge resistors. Screen printable conductor pastes based on palladium/silver and glass frit are described in U.S. Pat. No. 4,394,171 issued Jul. 19, 1983 to Kumaran M. Nair.